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INTERNATIONAL CHEMISTRY

OLYMPIADS

1986 - 1994

Problems and Solutions

edited by Marco Ziegler

Triple Medallist ICHO 1990, 1991, 1992

University of Fribourg, Switzerland

1993

by Marco Ziegler

This version is free of charge and may be distributed freely. Comments, corrections and other

intersting problems that will be incorprated in a larger problem compilation should be sent to

[email protected]. Table of Contents

Problems

1. Analytical chemistry p. 3


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2. Inorganic chemistry p. 5

3. Physical chemistry p. 12

4. Technical chemistry p. 19

5. Organic chemistry p. 21

6. Biochemistry p. 29

Solutions

1. Analytical chemistry p. 32

2. Inorganic chemistry p. 34

3. Physical chemistry p. 41

4. Technical chemistry p. 47

5. Organic chemistry p. 53

6. Biochemistry p. 62


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Problems

1. Analytical chemistry

IChOVeszprm - Budapest 1987

0 2.47 10.00?v(cm )3

emf

(V)

AB

C

Fig. 1 Potentiometric titration curve

25.00 ml of a neutral solution containing potassium chloride and potassium cyanide are potentiometrically titrated with

a standard 0.1000 molar silver nitrate solution at 25C using a silver electrode and a normal calomel half-cell with KNO3- salt bridge. The protonation of cyanid ions is negligible. The potentiometric curve obtained (emf (V)) vs. burette

readings (in ml) is shown in Fig. 1.

a) The end point of the reaction taking place during the titration are marked with A, B and C. Write the balanced ionic

equation for each reaction.

b) What volume of the titrant is required to reach point B ?

c) Calculate the concentrations of KCl and KCN in the sample solution.

d) Calculate the emf readings at the points A and C in volts.

e) What is the molar ratio Cl-/CN

-in the solution and in the precipitate at point C ?

Data:

Eo(Ag

+/Ag): 0.800 V

Eo(Calomel): 0.285 V

Ksp(AgCN): 10 -15.8 mol2/l2

2

2-

+ - 2

21.1 -2 2=[Ag(CN ) ]

[Ag ][CN ]= 10 mol l

Ksp(AgCl): 10-9.75

mol2/l

2

IChO Helsinki 1988

Chloride ions are analytically determined by precipitating them with silver nitrate. The precipitate is undergoing

decomposition in presence of light and forms elemental silver and chlorine. In aqueous solution the latter disproportionates

to chlorate (V) and chloride. With excess of silver ions, the chloride ions formed are precipitated whereas chlorate (V)ions arent.


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a) Write the balanced equations of the reactions mentioned above.

b) The gravimetric determination yielded a precipitate of which 12% by mass was decomposed by light. Determine the

size and direction of the error caused by this decomposition.

c) Consider a solution containing two weak acids HA and HL, 0.020 molar and 0.010 molar solutions respectively. The

acid constants are 110-4

for HA and 110-7

mol/l for HL. Determine graphically the pH of the solution using a

logarithmic diagram, log C = f(-log[H+]). Calculate the pH of the solution.

d) M forms a complex ML with the acid H2L with the formation constant K1. The solution contains an other metal ion

N that forms a complex NHL with the acid H2L. Determine the conditional equilibrium constant, K1 for the complexML in terms of [H

+] and K values.

[M] = total concentration of M not bound in ML

[L] = the sum of the concentrations of all species containing L except ML

1K =[ML]

[M][L]

1K =[ML]

[M ][L ]

NHL +K =

[NHL]

[N][L][H ]

In addition to K1, the acid constants Ka1 and Ka2 of H2L as well as the formation constant KNHL of NHL are known. You

may assume that the equilibrium concentration [H+] and [N] are known, too.

IChO Pittsburgh 1992

The minimum concentration of chloride ions needed in oyster beds for normal growth is 8 ppm (8 mg/l).

To 50.00 ml a sample of bay water a few drops of a K2CrO4-solution are added. The sample is then titrated with 16.16 ml

of a 0.00164 M AgNO3-solution when a bright red-orange precipitate starts to form.

a) What is the molar concentration of chloride in the sample ? Does the water contain sufficient chloride for the normal

growth of oysters ?

b) Write a balanced equation for the reaction of the analyte with the titrant.

c) Write a balanced net-ionic equation that describes the reaction responsible for the colour change at the endpoint of

the titration. Which compound produces the brick-red colour ?

d) The concentration of chromate at the endpoint is 0.020 M. Calculate the concentration of chloride in the solution

when the red precipitate is formed.

e) For this titration to work most efficiently, the solution being titrated must be neutral or slightly basic. Write a balanced

equation for the competing reaction that would occur in acidic medium influencing the observed endpoint of this

titration.

Typically, a buffer is added to the solution being titrated to control the pH. Suppose the pH of the sample of bay water

was 5.10, thus too acidic to perform the analysis accurately.

f) Select a buffer from the list that would enable you to establish and maintain a pH of 7.20 in aqueous medium. Show

the calculations which leads your choice.

Buffer systems Ka of weak acid

1. 0.1 M lactic acid / 0.1 M sodium lactate 1.4 10-4

2. 0.1 M acetic acid / 0.1 M sodium acetate 1.8 10-5

3. 0.1 M sodium dihydrogen phosphate / 0.1 M sodium hydrogen phosphate 6.2 10-8

4. 0.1 M ammonium chloride / 0.1 M ammonia 5.6 10-10

g) Using the selected buffer system, calculate the number of grams of weak acid and of conjugated base you would

need to dissolve in distilled water to prepare 500 ml of a stock solution buffered at a pH of 7.2.h) The chloride concentration in another 50.00 ml sample was determined by the Vollhard method. In this method an

excess of AgNO3 is added to the sample. The excess Ag+

is titrated with standardized KSCN, forming a precipitate of

AgSCN. The endpoint is signalled by the formation of the reddish-brown


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FeSCN2+

complex that forms when Ag+

is depleted. If the excess Ag+

from the addition of 50.00 ml of 0.00129 M

AgNO3 to the water sample required 27.46 ml of 1.41 10-3

M KSCN for titration, calculate the concentration of

chloride in the bay water sample.

In natural waters with much higher concentration of Cl-, it can be determined gravimetrically by precipitating them as

AgCl. A complicating feature of this method is the fact that AgCl is susceptible to photodecomposition as shown by the

reaction: AgCl (s) Ag (s) + Cl2 (g). Furthermore, if this photodecomposition occurs in the presence of excess Ag+,

the following additional reaction occurs:

3 Cl2 (g) + 3 H2O + 5 Ag+ 5 AgCl + ClO3

-+ 6 H

+

i) Will the apparent weight of AgCl be too high or too low ? Explain your answer showing by how many grams the two

values will differ.

data: Ksp for AgCl = 1.78 10-10

Ksp for Ag2CrO4 = 1.00 10-12

IChO Peruggia 1993

The reflux of bile and duodenal matter is suspected to be the major cause of gastritis and medical therapy is based on

treatment with antiacida that, by binding bile acids and lysolecithin, buffer the pH of gastric juces. The separation of two

bile acids, i.e. cholic acid (CA) and glycocholic acid (GCA) was achieved with high performance liquid chromatography

(HPLC). Two chromatographic columns (A and B, respectively) were selected on the basis of published literature data.

The retention times (t) of the two acids, of a substance not retained and of a compound used as internal standard (i.s.)

are shown on the Table 1. Both columns are 25 cm long and show an efficiency of 2.56 104 theoretical plates per meter(N/m). In actual analysis, an artificial gastric juice was extracted with appropriate solvents and the final solution (1mL)

contained 100% of the two acids present in the original mixture. 100 L of the extract added with 2.7 L of internalstandard were analyzed by HPLC using the selected column. The response factors (F) of CA and GCA with respect to the

i.s. and chromatographic peak areas of the two compounds are reported in the Table given below.

Column A

t (sec) Column B

t (sec)

F

Area

Unretained compound 120 130

Cholic Acid (CA) 380 350 0.5 2200

Glycocholic Acid (GCA) 399 395 0.2 3520

Internal standard (i.s.) 442 430 2304

Note that:

[ ] [ ]R N K K= +( / ) ( ) / '/( ' )4 1 1

= = t t t t t t2 1 2 0 1 0' '/ ( ) / ( )

K t t t t t' / ( ) / '= = 2 0 2 0 0

a) Using R, and K values, demonstrate which of the two chromatographic columns would be best to use, consideringthat an accurate determination requires the best possible (baseline) separation of all compounds (do not take into

account the unretained compound) ?

b) Calculate the amounts of each acid in the extract solution.

IChO Oslo 94

Lactic acid is formed in the muscles during intense activity (anaerobic metabolism). In the blood, lactic acid is neutralized


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by reaction with hydrogen carbonate ions. This will be illustrated by the following calculations:

Lactic acid, written HL, is monoprotic, and the acid dissociation constant is KHL

= l.4 x l0-4.

The acid dissociation constants for carbonic acid are: Ka1

= 4.5 x 10-7 and Ka2

= 4.7 x 10-11. All carbon dioxide remains

dissolved during the reactions.

a) Calculate the pH of a 3.00 x 10-3 M solution of HL.

b) Calculate the value of the equilibrium constant for the reaction between lactic acid and hydrogen carbonate ions.

c) 3.00 x 10-3 mol of lactic acid (HL) is added to 1.00 L of 0.024 M solution of NaHCO3

(no change in volume, HL

completely neutralized). Calculate the pH of the solution of NaHCO3

before HL is added. Calculate the pH of the

solution after the addition of HL.

d) The pH of the blood in a person changed from 7.40 to 7.00 due to the lactic acid which formed during physical

activity. Let an aqueous solution having pH = 7.40 and [HCO3

-] = 0.022 M represent blood in the following calculation

and in f). Calculate the number of moles of lactic acid which must be added to 1.00 L of this solution so that the pH

becomes7.00.

e) In a saturated aqueous solution of CaCO3

(s) the pH is measured as 9.95. Calculate the solubility of calcium carbonate

in water and show that the calculated value for the solubility product constant Ksp

is 5 x 10-9.

f) Blood contains calcium ions. Calculate the maximum concentration of free calcium ions in the solution given in d).

(pH = 7.40, [HCO3

-] =0.022 M)

2. Inorganic chemistry

IChO Leiden 1986

Compounds containing divalent platinum with the general formula PtX2 (amine)2 (X = Cl2, SO42-

, malonate, etc.) have

met a lot of scientific interest because of their biological activity, particularly in view of their properties in the treatment

of tumours. The best known compound used clinically is PtCl2(NH3)2. This compound, in which platinum is coordinated

in a planar square, has two geometrical isomers of which only one shows the antitumour activity.

a) Sketch the spatial structures of the two possible isomers.

b) How many isomers has PtBrCl(NH3)2 ? Sketch all of them.

It is possible to replace the amine ligands by one ligand containing two donor atoms (N). Then one obtains a chelating

ligand, such as 1,2-diaminoethane (en).

c) Show graphically that PtBrCl(en) has only one stable structure.

The ligand en can be substituted via methylation to form dmen or pn (racedmic).

NH2N NH2Nor

dmen pn (racemic)

d) Give spatial structures of all isomers of the following compounds: PtCl2(dmen), PtCl2(pn), PtBrCl(dmen) and PtBrCl(pn).


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NH2N NH2Nor

dmen pn (racemic)

These compounds can isomerize in aqueous solution through dissociation of a ligand and transient replacement of the

stronger ligands by the weak ligand water. Cl-and Br

-are replaced relatively easily, but it is more difficult to replace the

amine ligands, which usually requires heating.

e) Considering each of the isomers in the previous questions a-d, indicate which isomers can be converted to another

at room temperature. Give both the original molecule and the products.

f) PtCl2(en) reacts with Br-in a molar proportion of 1:2 at room temperature. Which compound would you expect to form

in what proportion ? You can assume that the Pt-Br and Pt-Cl bonds are equally strong and that there is no perturbing

influence from hydrolysis.

N

NN

N

Pt

sugar

O

H2N

H2O

NH3H3N

g) Using the equation to express chemical equilibrium, show that hydrolysis hardly ever occurs in blood but that is doesoccur in the cells. Note: PtCl2(NH3)2 hydrolyses to Pt(NH3)2(H2O)2

2+and 2 Cl

-. In cells the Cl

-concentration is low,

in blood it is fairly high.

After hydrolysis in the tumour cell a reactive platinum ion is formed to which two NH3 groups are still bound, which were

found in the urine of patients treated with this compound. The reactive platinum ion appears to be bound to cellular

DNA, where the bonding occurs via guanine to one of the N-atoms.

As a result of the two reactive sites of platinum and the two unreactive NH3 ligands, it can form a second bond to DNA

in addition. Research has shown that this happens in particular with a second guanine base from the same strand of

DNA.

h) Show by calculation which of the two isomers in question a) can form this bond. (note: Pt-N distance - 210 pm, DNAbase distance = 320 pm).

IChO Leiden 1986

The compound Na5P3O10 is used as an additive for detergents to bind the Ca2+

and Mg2+

ions present in water in order

to prevent the precipitation of their fatty acid salts on the laundry.

a) Draw the structure of the ions (P3O10)5-

and (P3O9)3-

assuming that P-P bonds do not occur.

b) Assuming an octahedral coordination of the Mg2+

ion give a drawing of the Mg(P3O10)(H2O)n)3-

ion also indicating

the value for n.

The complex ions of Mg2+ and Ca2+ and triphosphate are well soluble in water. They are, among other things, responsiblefor the wild growth of algae in surface waters. They could be removed by precipitation as an insoluble compound.


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c) Give some possibilities (ions) to precipitate the triphosphates bound to Ca2+

or Mg2+

.

d) Calculate the number of grams of Na5P3O10 necessary in a washing machine of 20 liters to reduce the amount of Ca2+

in city water (0.225 g/l) to an acceptable maximum of 0.02 g/l. Effects of the pH, the precipitation of Ca(OH)2 and

possible effects by other positive ions can be neglected. The following data is given:

1

2+3 10

5-

3 103-

-6K =[Ca ][P O ]

[Ca P O ]

= 1.0 10

Molar mass of Na5P3O10 is 366 g, the atomic mass of Ca is 40 g/mol.


500 mg of a hydrated sodium salt of phosphoric acid are dissolved in 50.0 ml of 0.1 molar sulphuric acid. This solution is

diluted with distilled water to 100.0 ml and 20.0 ml thereof are titrated with 0.100 molar NaOH solution using thymolphthalein

as indicator. The average of the burette reading is 26.53 ml. The pH at the endpoint is 10.00. The cumulative protonation

constants are given by

nn 4

n-3

43- + n

K =[H PO ]

[PO ][H ]

where pK1 = -11.70; pK2 = -18.6; pK3 = -20.6. Relative atomic masses are: Na = 23.0 and P = 31.0

a) Calculate the percentage distribution, by moles, of all protonated HnPO4n-3

species at the end point.

b) What is the stoichiometric formula of the salt ?

IChO Helsinki 1988

Upon heating of a mixture of A and fluorine (molar ratio 1:9, high pressure) to 900 C three compounds (B, C and D) areformed. All three products are crystalline solids with melting points below 150 C. The fluorine content of C is found tobe 36.7% and that of D 46.5% (by weight). When B is treated with anhydrous HOSO2F at -75C a compound E is formed:B + HOS2F E + HFE is a solid which is stable for weeks at 0C, but decomposes in days at room temperature. The electron densitydistribution of E obtained through X-ray diffraction studies is shown on two intersecting, mutually perpendicular planes

(see Fig. 2). The numbers indicated on the maps relate to the electron density in the neighbourhood of the atoms of E as

a function of the spatial coordinates. The maxima found in these maps coincide with the locations of the atoms and the

values are approximately proportional to the number of electrons in the atom in question.

a) Show where the maxima lie by drawing the contour curves around the maxima, connecting points of equal electron

densities. Label each maximum to show the identities of the atoms in E.

b) When 450.0 mg of C was treated with an excess of mercury, 53.25 ml of A was liberated at a pressure of 101.0 kPa at a

temperature of 25C. Calculate the relative atomic mass of A.c) Identify A, B, C, D and E.

d) Use the valence-shell electron-pair repulsion theory (VSEPR) to propose electron-pair geometries for B and C. Using

the two electron density maps, sketch the molecular geometry of E.

The original mixture was hydrolysed in water. B reacts to A while liberating oxygen and producing aqueous hydrogen

fluoride. Hydrolysis of C leads to A and oxygen (in molar ratio of 4:3) and yields an aqueous solution of AO 3 and

hydrogen fluoride. D hydrolyses to an aqueous solution of AO3 and hydrogen fluoride.

e) Write the equations for the three hydrolysis reactions.

f) Quantitative hydrolysis of a mixture of B,C and D gives 60.2 ml of gas (measured at 290 K and 100 kPa). The oxygen

content of this gas is 40.0% (by volume). The amount of AO3 dissolved in water is titrated with an aqueous 0.1 molar

FeSO4 solution and 36.0 ml used thereby. During the titration Fe

2+

is oxidized to Fe

3+

and AO3 is reduced to A.Calculate the composition (% by moles) of the original mixture of A, B, C and D.


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Y-direction

0

100

200

300

400

500

600

-100

-200

(pm)

1 2 2 1 1 2 2 2 1 2 1 2 1 2

1 2 1 2 1 1 2 1 3 1 1 2 1 2

2 2 1 1 2 1 2 4 26 3 1 2 1 2

3 1 2 1 2 2 3 9 32 11 2 2 1 2

1 2 1 1 2 2 4 20 58 21 3 1 2 1

2 2 1 1 2 1 3 10 33 11 4 2 2 2

1 1 2 2 1 2 2 8 19 6 3 3 1 1

1 1 2 1 2 2 5 3 6 3 2 1 2 1

1 2 2 1 4 17 20 16 3 2 2 3 1 2

1 1 2 4 12 33 76 36 13 4 3 1 2 1

2 1 2 6 36 100 220 101 31 6 1 2 1 2

1 2 4 13 61 149 356 152 62 15 6 1 2 1

2 2 1 6 31 105 232 90 26 7 3 1 1 2

1 1 2 6 12 36 65 28 10 4 2 1 1 2

2 2 1 2 7 15 15 13 6 1 2 1 1 2

1 1 1 3 5 3 6 2 1 1 1 2 2 1

1 2 2 6 14 6 3 1 1 2 2 2 1 2

2 2 3 11 23 10 3 8 2 1 2 1 2 2

2 2 7 19 32 16 2 1 2 2 1 1 2 2

1 2 3 9 25 12 3 2 2 2 2 1 1 2

2 2 1 4 13 5 6 4 2 1 1 1 2 1

1 1 1 2 6 10 33 11 1 2 1 1 1 2

1 2 1 4 11 22 80 23 5 2 1 2 1 2

1 2 9 8 30 52 104 49 16 1 2 3 1 2

1 1 2 3 12 21 75 23 6 2 1 2 1 1

1 2 1 2 4 9 36 11 2 2 1 1 2 2

1 1 2 4 15 5 6 4 1 1 2 1 1 2

2 2 3 11 24 18 4 2 1 2 1 2 1 21 2 6 17 52 19 6 3 1 2 1 2 2 1

2 1 3 8 23 8 4 1 2 1 2 1 1 2

2 2 1 4 13 5 2 2 1 1 3 2 1 2

1 1 2 1 2 1 1 1 2 1 1 2 2 1

1 2 1 1 2 2 1 2 2 1 1 2 2 1

0 100 200 300-100-20000

X direction (pm)

Z-direction


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0

100

200

300

400

-100

-200

(pm)

-300

-400

1 2 1 1 2 2 2 1 1 1 2 2 2 2

1 2 1 1 2 1 1 2 2 2 1 2 1 1

1 2 2 1 1 1 2 2 1 1 1 2 2 1

1 1 2 2 2 1 2 1 1 1 1 2 1 1

1 2 2 2 1 2 1 1 2 1 2 1 1 2

1 1 2 2 2 1 1 2 1 2 1 2 1 1

2 2 2 2 1 1 2 2 1 1 2 2 2 1

1 1 2 1 1 1 2 2 2 1 1 2 2 1

2 2 1 1 1 2 3 2 1 1 2 2 1 1

1 2 2 1 1 2 2 1 2 2 2 1 1 12 2 2 2 1 1 2 1 1 2 2 3 1 2

1 1 1 2 1 1 2 4 11 3 2 1 1 1

1 1 2 2 1 1 1 8 21 9 3 2 1 2

1 2 1 1 2 3 4 10 50 19 3 2 1 2

1 1 2 2 2 5 6 10 23 9 2 1 1 2

1 1 1 2 4 8 33 8 10 3 1 1 2 1

1 2 1 3 9 21 74 22 5 1 2 2 2 2

1 2 9 8 20 58 194 49 16 1 2 3 1 2

1 1 3 2 11 23 81 23 13 1 1 1 1 2

1 2 1 3 3 12 37 11 4 2 2 2 1 2

1 1 1 2 1 3 7 5 15 4 2 3 1 1

2 1 2 2 2 2 4 10 24 9 3 1 2 2

1 1 2 1 2 3 8 19 52 18 3 2 2 2

2 2 2 1 1 2 3 9 21 8 3 2 2 1

3 2 1 1 2 1 2 4 12 3 1 1 1 1

2 1 2 2 1 1 2 2 2 1 2 2 2 2

1 2 2 2 1 1 1 2 2 1 2 1 1 2

2 2 1 1 2 2 2 2 1 1 2 2 1 2

1 2 1 1 2 1 1 2 1 1 2 2 2 2

2 1 1 2 2 2 1 3 1 2 2 3 3 1

1 1 1 2 2 2 2 1 3 2 1 1 2 12 1 1 2 2 2 1 1 2 1 2 1 1 1

1 2 1 1 2 2 2 2 1 1 2 2 2 1

0 100 200 300-100-200-300

X-direction (pm)

Fig. 2 Electron density distribution of compound E

IChO Halle 1989

To determine the solubility product of copper(II) iodate (Cu(IO3)2 by iodometric titration in an acidic solution (T=25 C)

20.00 ml of a saturated aqueous solution 30.00 ml of a 0.10 molar sodium thiosulphate solution are needed.

a) Write the sequence of balanced equations for the above described reactions.


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b) Calculate the initial concentration of Cu2+

and the solubility product of copper(II) iodate. Activity coefficients can be

neglected.

IChO Halle 1989

NP

OCl

Cl

OCl

Cl NP P

OCl

Cl

ClCl

Cl

+ PCl5 + POCL3 + HCl

I II III IV

N

P

OCl

Cl

OCl

Cl N

P P

OCl

Cl

ClCl

Cl

+ PCl5 + POCL3 + HCl 32P labelled

phosphorus pentachloride (half-life t1/2 = 14.3 days) is used to study the electrophilic attack of a PCl 4+

cation on

nitrogen or on oxygen. The reaction is carried out in CCl4 and the solvent and product IV distilled off.

Samples of III (remaining in the distillation flask), of IV (in the distillate) and samples of the starting material II are

hydrolyzed by heating with a strong sodium hydroxide solution. The phosphate ions formed are precipitated as ammonium

magnesium phosphate. Purified samples of the three precipitates are then dissolved by known volumes of water and the

radioactivity measured.

a) Write the balanced equations for the reaction of red phosphorus forming PCl5b) Write the reaction equations for complete hydrolysis of the compounds II and III using sodium hydroxide.

c) How long does it take in order to lower the initial radioactivity to 10-3

of the initial value?

d) Write two alternative mechanisms for the reaction of labelled PCl4+ with the anion of I.e) After hydrolysis the precipitated ammonium magnesium phosphates show the following values for radioactivity:

II. 2380 Bq for 128 mg of Mg(NH4)PO4III. 28 Bq for 153 mg of Mg(NH4)PO4IV. 2627 Bq for 142 mg of Mg(NH4)PO4

Using these data, what can you say about the nucleophilic centre attacked by PCl4+?

Data:

H3PO4: pK1 = 2.2, pK2 = 7.2, pK3 = 12.4

solubility product of Mg(NH4)PO4:pKsp = 12.6

equilibrium concentration of NH4+

= 0.1 mol/l

f) Calculate the solubility for Mg(NH4)PO4 at pH equal to 10 under idealized conditions (activity coefficients can be

neglected).

IChO Paris 1990

The mineral apatite is a mixture of tricalcium diphosphate Ca3(PO4)2, calcium sulphate, calcium fluoride, calcium carbonate

and silica. An elemental analysis gave the following result:

CaO P2O5 SiO2 F SO3 CO2% by mass 47.3 28.4 3.4 3.4 3.5 6.1

A sample of m0 g of this mineral is treated with 50.0 ml of a solution containing 0.500 mol/L phosphoric acid and 0.100


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mol/l sulphuric acid. The mixture is completely dehydrated by heating up to 70 C. Toxic gases are emitted thereby. m1g of dry residue is obtained. In these conditions, only phosphate Ca(H2PO4)2 is formed, silica and silicate do not react.

1.00 g of this residue is treated with 50.0 ml of water at 40 C, filtered, dried and m2 g obtained. This new residue is mainlycontaining gypsum (CaSO42H2O) whose solubility between 20 C and 50 C is equal to 2.3 g/l.

a) Write the balanced equations for all reactions. From what mass of apatite should one start if the reactions are

stoichiometric?

b) Starting with m0 g of apatite, m1 = 5.49 g of residue are obtained. What mass should theoretically have beenobtained? This result is due to the presence of two products that are not expected to be in the residue. Which

product are these?

Traditionally, the yield is expressed as percentage of oxide. The phosphorous content is expressed as if it were P2O5. If

n2 is the amount of soluble product obtained, n1 the amount of substances added as acid, n0 the amount of apatite

added, the yield is:

expr = 100n

n + n

2

1 0

c) m2 = 0.144 g of residue is obtained on the filter. Calculate rexp.d) The experimental yield is over 100 %. Calculate a value of r nearer to the real yield.

IChO Paris 1990

a) Consider a 1.0010-2

solution of copper(II) nitrate. The pH of this solution is 4.65. Give the equation for the formation

of the conjugate base of the hydrated Cu2+

ion and calculate the pKa of the corresponding acid-base pair. The

solubility product of copper(II) hydroxide is Ksp = 10-20

. At what pH would Cu(OH)2 precipitate from the solution

under consideration? Justify your calculation, showing that the conjugate base of this hydrated Cu2+

ion is present

in negligible quantity.

b) Write down the equation for the disproportionation of copper(I) ions and calculate the corresponding equilibrium

constant. E10(Cu+/Cu) = 0.52 V, E20(Cu2+/Cu+) = 0.16 V. Calculate the composition in mol/l of the solution obtainedon dissolving 1.010

-2mol of copper(I) in 1.0 l of water. Name two chemical species which also disproportionate in

aqueous solution; write down the equations and describe the experimental conditions under which disproportionation

is observed.

c) Consider the stability of copper(I) oxide (Cu2O) in contact with a 1.0010-2

mol/l solution of Cu2+

ions. The stability

product of copper(I) oxide is Ksp = [Cu+][OH

-] = 10

-15

Quote a simple experiment allowing the observation of the precipitation of Cu2O.

d) The dissociation constant of the complex ion [Cu(NH3)2]+

is KD = 10-11

. Calculate the standard electrode potential

of the couple:

[Cu(NH ) ] + e Cu + 2 NH3 2+ -

3

e) The emf of the couple [Cu(NH3)4]2+ + 2e- _ Cu = 4 NH3 is E3

0 = - 0,02 V. Calculate the dissociation constant of the

complex ion [Cu(NH3)4]2+

. Deduce from it the emf of the couple:

[Cu(NH ) ] + e [Cu(NH ) ] + 2 NH3 42+ -

3 2+

3

Does the disproportionation of the cation [Cu(NH3)2]+

take place?

IChO Lodz 1991

a) Show that 0.1 mol of Tl2S dissolves in a 1M solution of any strong monoprotic non-coordinating acid.b) Show that 0.1 CuS dissolves in a 1M HNO3 but not in a 1M HCl solution.


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Information:

Assume that Cu2+

ions dont form stable complexes with chloride ions in aqueous solutions.

E0(S/S

2-) = - 0.48 V E

0(NO3

-/NO(aq)) = 0.96 V

pKa(H2S) = 7 pKa(HS-) = 13

Ksp(Tl2S) = 10-20

Ksp(CuS) = 10-35

R = 8.314 J K

-1

mol

-1

F = 96487 C mol

-1

IChO Lodz 1991

Type II electrodes are made of a metal covered with a sparingly soluble salt of the metal that are dipped into a soluble salt

solution containing an anion of the sparingly soluble salt. The silver/silverchloride (Ag, AgCl/Cl-) and the calomel

electrode (Hg, Hg2Cl2/Cl-) are examples of such electrodes. The standard emf of a cell built of those electrodes (-)

Ag,AgCl/Cl-/Hg2Cl2/Hg (+) is E

0= 0.0455 V. at T = 298 K. The temperature coefficient for this cell is dE

0/dT = 3.3810

-4

V/K.

a) Give the equations of the reactions taking place at both the cell electrodes and the overall cell reaction.b) Calculate the Gibbs free energy change (G

0) for the process taking place in the cell. What does its sign imply ?

c) Calculate the enthalpy change for the process taking place at 298 K, given S = n F dE/dT.d) Knowing the standard potential of Ag/Ag

+electrode is E

0= 0.799 V and the solubility product of AgCl Ksp = 1.7310

-

10, calculate the standard electrode potential value of the silver/silver chloride electrode. Derive an expression

showing the dependence between E0

(Ag/Ag+) and E

0(Ag, AgCl/Cl

-).

e) Calculate the solubility product of Hg2Cl2 knowing that the standard potential of the calomel electrode is E0

= 0.798

V.


Many streams drain in areas where coal or metallic ores are mined. These streams have become acidic and contain high

concentrations of dissolved iron and sulfate, due to sulfur-containing ores being exposed to the atmosphere or to

oxygenated waters. The most common sulfur-containing mineral is pyrite, FeS2, in which the oxidation state of iron is +2.

As the iron-rich streams mix with other waters, the dissolved iron precipitates as goethite, FeO(OH), which coats the

stream bottom while the water remains acidic.

a) Draw the electron dot structure that illustrates the bonding in the S22-

-ion, showing all valence electrons.

b) Write a balanced chemical equation to show how hydrogen ions (H+) are genrated during the oxidation of pyrite to

form iron(II) ions and sulphate.

c) Write a balanced equation to show how many additional moles of hydrogen are generated when iron(II)ions are

oxidized to form the mineral goethite, FeO(OH).

d) Calculate how many moles of pyrite would be required to bring 1.0 litre of pure water to a pH of 3.0 if the pyrite was

completely converted into FeO(OH) and H+. Neglect the formation of HSO4-.

e) The concentration of iron as Fe(II) in a stream is 0.00835 M. At a very narrow point in the stream it empties into a large

pond, with a flow rate of 20.0 l/min. The water in this stream is sufficiently aerated that 75% of the iron(II) is oxidized

to Fe(III) which immediately precipitates as Fe(OH)3 later aging to became Fe2O3. What mass of Fe2O3 will be

deposited on the bottom of the pond in two years ?


Diatoms, microscopic organisms, produce carbohydrates from carbon dioxide and water by normal photosynthesis: 6

CO2 + 6 H2O + solar energy C6H12O6 + 6 O2

During the first five years of life whales gain 75 kg of mass per day by feeding on krill. The whale must consume ten timesthis mass of krill each day. The krill must consume 10.0 kg of diatoms to produce 1.0 kg of krill.

a) Assuming that the mass gain in the first years of a whales life is due to the consumption of carbohydrates, calculate

the volume of CO2 at 0 C and 101kPa that must be used by the diatoms to produce the carbohydrates consumed by


14/40

a blue whale in its first five years of life.

b) There is 0.23 mL of dissolved CO2 per l sea water (at 24 C and 101kPa). If diatoms can completely remove carbondioxide from the water they process, what volume of water would they process to produce the carbohydrates

required by a blue whale during the first five years of life ?

c) 3% of the mass of a 9.1104

kg adult whale is nitrogen. What is the maximum mass of NH4+

that can become available

for other marine organisms if one adult whale dies ?

d) 18% of a adult whales mass is carbon which can be returned to the atmosphere as CO2 being removed from there by

weathering of rocks containing calcium silicate.

CaSiO3 (s) + 2 CO2 + 3 H2O (l) Ca2+

(aq) + 2 HCO3-(aq) + H4SiO4 (aq)

What are the maximum number of grams of CaSiO3 that can be weathered by the carbon dioxide produced from the

decomposition of 1000 blue whales, the number estimated to die annually ?


The Pourbaix diagrams of water, nitrogen and manganese are depicted in Fig. 3.

a) Which species of nitrogen is predominant in a1) O2-rich lakes of pH ~ 6 , a2) O2-depleted lakes of pH ~ 3 b)Whichspecies of manganese is predominant in b1) O2-rich lakes of pH ~ 6, b2) highly O2-depleted lakes of pH ~ 12 ?

c) People often find that clear, slightly acidic (pH ~ 5) water drawn from wells deposits a black manganese-containing

solid on standing in toilet bowls. What is the chemical formula of the black solid ? What is the corresponding species

of manganese found in well water while it is still underground ?

d) Which of the two species of nitrogen oxidize Mn (s) to Mn2+

(aq) according to the Pourbaix diagrams ? Which one

of this actually (in practice) does not oxidize Mn (s) at room temperature ?

e) Which of the following compounds oxidize NH3 (aq) or NH4+

(aq) to N2 (g) ?

Mn, Mn(OH)2, Mn2+

, Mn3O4, Mn2O3, MnO2, MnO42-

, MnO4-

We shall discuss the properties of the two salts ammonium permanganate, NH4MnO4, and ammonium manganate,

(NH4)2MnO4, which is a rarely known salt.

f) Are the salts NH4MnO4 and (NH4)2MnO4 thermodynamically stable ?

g) Write the balanced equation of the decomposition of both salts to give MnO2, Mn and N2.

h) Is it dangerous to grind together h1) potassium nitrate and manganese metal, h2) KNO3 and MnO2 ?

i) The standard reduction potential for the reduction of MnO4-

to MnO2 is 1.692 V. Applying the Nernst equation,

calculate the reduction potential for the reduction of 0.00100 M MnO4-at a pH = 4.0.


15/40

Fig. 3 Pourbaix diagrams of H2O, N and Mn

IChO Peruggia 93131I is a radioactive isotope of iodine (e- emitter) used in nuclear medicine for analytical procedures to determine thyroid

endocrine disorders by scintigraphy. The decay rate constant, k, is 9.93 10-7

s-1

.

a) Write the decay reaction of131I

b) Calculate the half-life of131I expressed in days

c) Calculate the time necessary (expressed in days) for a sample of131I to reduce its activity to 30% of the

original value.

d) Knowing that a Geiger counter detects activities of the order of 10-4c, calculate the the minimum amount of131I which could be detected by this counter. Note that 1 Curie (c) is the amount of radioisotopes that produces 3.7

1010 disintegrations per second.

IChO Oslo 1994

Sulfur forms many different compounds with oxygen and halogens (sulfur as the central atom). These compounds are

mainly molecular, and many are easily hydrolysed in water.


16/40

a) Write Lewis structures for the molecules SCl2, SO

3, SO

2ClF, SF

4, and SBrF

5. Show all non-bonding electrons.

b) Carefully draw the geometries of the same 5 molecules. (Disregard small deviations from ideal angles.)

c) A compound, consisting of sulfur (one atom per molecule), oxygen, and one or more of the elements F, Cl, Br, and I,

was examined. A small amount of the substance was reacted with water. It was completely hydrolyzed without any

oxidation or reduction, and all reaction products dissolved. 0.1M solutions of a series of test reagents were added to

separate, small portions of a diluted solution of the substance.

State which ions were being tested for in the following tests:

I) Addition of HNO3and AgNO

3.

ii) Addition of Ba(NO3)

2.

iii) Adjustment to pH = 7 with NH3and addition of Ca(NO

3)

2

Write the equations for the possible reactions in the following tests:

iv) Addition of KMnO4

to an acidified solution of the substance followed by addition of Ba(NO3

)2

v) Addition of Cu(NO3)

2.

d) In practice, the tests in c) gave the following results:

i) A yellowish precipitate

ii) No precipitate

iii) No visible reaction

iv) The main features were that the characteristic colour of KMnO4disappeared, and a white precipitatew a s

formed upon subsequent addition of Ba(NO3)

2

v) No precipitate

Write the formulae of the possible compounds, taking the results of these tests into account.

e) Finally, a simple quantitative analysis was undertaken: 7.190 g of the substance was weighed out and dissolved in

water to give 250.0 mL solution. To 25.00 mL of this solution, nitric acid, and enough AgNO3

to secure complete

precipitation, was added. The precipitate weighed 1.452 g after washing and drying. Determine the formula of the

compound.

f) Write the equation describing the reaction of the compound with water.

If you have not arrived at a formula for the compound, use SOClF for this equation.

IChO Oslo 1994

a) Nitrogen in agricultural materials is often determined by the Kjeldahl method. The method involves a treatment of the

sample with hot concentrated sulphuric acid, to convert organically bound nitrogen to ammonium ion. Concentrated

sodium hydroxide is then added, and the ammonia formed is distilled into hydrochloric acid of known volume and

concentration. The excess hydrochloric acid is then back-titrated with a standard solution of sodium hydroxide, to

determine nitrogen in the sample.

Calculate the percentage by mass of nitrogen in the sample.

b) Calculate the pH of the solution during the titration in a), after 0 mL, 9.65 mL, 19.30 mL and 28.95 mL of sodium

hydroxide have been added. Disregard any volume change during the reaction of ammonia gas with hydrochloric

acid.

Ka for ammonium ion is 5.7 x 10-10

c) Draw the titration curve based on the calculations in b).


17/40

d) State the pH range of the indicator which could be used for the back titration.

e) The Kjeldahl method can also be used to determine the molecular weight of amino acids. In a given experiment, the

molecular weight of a naturally occurring amino acid was determined by digesting 0.2345 g of the pure acid, and

distilling the ammonia into 50.00 mL of 0.1010 M hydrochloric acid. A titration volume of 17.50 mL was obtained for

the back titration with 0.1050 M sodium hydroxide. Calculate the molecular weight of the amino acid based on one

and two nitrogen groups in the molecule, respectively.

3. Physical chemistry

IChO Leiden 1986

In order to explain why dyes are coloured, they can be considered as rod-like, one-dimensional molecules over which the

electrons are distributed. The wave lengths of the electrons should fit to the available space, which is the length l. When

absorbing light, an electron makes a transition from a lower to a higher energy state. The energy difference is given by:

E =h c

where =h

p

a) Give a general expression for possible wavelengths of the electron as a function of the length l.

In the particle in the box model, only the variations in the kinetic energy of the electrons are considered.

b) Give an expression for the possible energies that electrons in the molecule can have (as a function of l).

c) Show that for a chain of length l with k electrons, the longest wavelength absorption occurs at:

=8mcl

h(k +1)

for even values of k2

d) Derive an expression for the wavelength of the first electronic transition as a function of the number of C-atoms for

even values of n.

e) Calculate the minimum number of C-atoms (conjugated systems) to obtain a visible colour. C-C bond length is 142 pm.

The retina in the human eye contains rhodopsin, a light absorbent.

O2

3 4 5

6

7

89

10

11

1213

14

15

1

59 39

The C-atoms 7 through 12 are planar. The angle between the bonds C5-C6, C7-C8, C11-C12 and C13-C14 is about 39.According to the particle in the box theory fragment C7 through C12 should absorb at about 213 nm. In reality the

absorption of retinal occurs at 308 nm.

f) Give a reason for the longer wavelength that is observed in praxis using the above mentioned theories.

g) When retinal is bound to opsin to form rhodopsin, the absorption occurs around 600 nm. Which atoms must be

forced into one plane by the protein ? Show by calculation that its true.


18/40

IChO Helsinki 1988

A typical family car has four cylinders with a total cylinder volume of 1600 cm3

and a fuel consumption of 7.0 l per 100 km

when driving at a speed of 90 km / h. During one second each cylinder goes through 25 burn cycles and consumes 0.4

g of fuel. Assume that fuel consists of 2,2,4 - trimethyl - pentane, C8H18. The compression ratio of the cylinder is 1:8.

a) Calculate the air intake of the engine (m3/s). The gasified fuel and air are introduced into the cylinder when its volume

is largest until the pressure is 101.0 kPa. Temperature of both incoming air and fuel are 100 C. Air contains 21.0 % (byvolume) of O2 and 79.0 % of N2. It is assumed that 10.0 % of the carbon forms CO upon combustion and that nitrogen

remains inert.

b) The gasified fuel and the air are compressed until the volume in the cylinder is at its smallest and then ignited.

Calculate the composition (% by volume) and the temperature of the exhaust gases immediately after the combustion

(exhaust gases have not yet started to expand). The following data is given:

Compound Hf(kJ / mol) Cp (J / mol K)

O2(g) 0.0 29.36

N2(g) 0.0 29.13

CO(g) -110.53 29.14

CO2(g) -395.51 37.11

H2O (g) -241.82 33.58

2,2,4 - trimethylpentane -187.82

c) Calculate the final temperature of the leaving gases assuming that the piston has moved to expand the gases to the

maximum volume of the cylinder and that the final gas pressure in the cylinder is 200 kPa.

d) To convert CO(g) into CO2(g) the exhaust gases are led through a bed of catalysts with the following work function:

n(CO)

n(CO )=

1

4k

n(CO)

n(CO )v e

2 12

-T

T0

where [n(CO) / n(CO2)]1 is the molar ration before the catalyst, v is the flow rate in mol / s and T the temperature of the

gases entering the catalyst (the same as the temperature of the leaving exhaust gases). T0 is a reference temperature (373K) and k is equal to 3.141 s / mol. Calculate the composition (% by volume) of the exhaust gases leaving the catalyst.

IChO Helsinki 1988

The periodic system of the elements in our three-dimensional world is based on the four electron quantum numbers n =

1,2,3,....; l = 0,1,....,n-1, m = 0, 1, 2,...., 1; and s = 1/2. In Flatlandia, a two-dimensional world, the periodic system is

thus based on three electron quantum numbers: n = 1,2,3,...; m1 = 0, 1, 2, ...., (n-1); and s = 1/2 where m1 plays the

combined role of l and m of the three dimensional world. The following tasks relate to this two-dimensional world, where

the chemical and physical experience obtained from our world is supposed to be still applicable.

a) Draw the first four periods of the Flatlandian periodic table of the elements. Number them according to their nuclear

charge. Use the atomic numbers (Z) as symbols of the specific element. Write the electron configuration for each

point.

b) Draw the hybrid orbitals of the elements with n = 2. Which element is the basis for the organic chemistry in Flatlandia?

Find the Flatlandian analogous for ethane, ethene and cyclohexane. What kind of aromatic ring compounds are

possible ?

c) Which rules in Flatlandia correspond to the octet and the 18-electron rules in the three dimensional world?

d) Predict graphically the trends in the first ionization energies of the Flatlandian elements with n = 2. Show graphically

how the electronegativities of the elements increase in the Flatlandian periodic table.

e) Draw the molecular orbital energy diagrams of the neutral homonuclear diatomic molecules of the elements with n =

2. Which of these molecules are stable in Flatlandia?

f) Consider simple binary compounds of the elements (n=2) with Z=1. Draw their Lewis structure, predict their geometries

and propose analogs for them in the three dimensional world.

g) Consider elements with n3. Propose an analog and write the chemical symbol from our world for each of theseFlatlandian elements. On the basis of this chemical and physical analog predict which two-dimensional elements are

solid, liquid or gaseous at normal pressure and temperature.


19/40

IChO Halle 1989

A mixture of gases containing mainly carbon monoxide and hydrogen is produced by the reaction of alkanes with steam:

4 2 2-1

4 2 2-1

CH + 0.5 O CO +2 H H = - 36 kJmol

CH + H O CO + 3H H = 216 kJmol

a) Using equations (1) and (2) write down an overall reaction (3) so that the net enthalpy change is zero.

b) The synthesis of methanol from carbon monoxide and hydrogen is carried out either

1. in two steps, where the starting mixture corresponding to equation (3) is compressed from 0.1106

Pa to 3 106

Pa,

and the mixture of products thereof compressed again from 3 106

Pa to 6 106

Pa

or

2. in one step, where the mixture of products corresponding to equation (3) is compressed from 0.1 106 Pa to 6 106

Pa.

Calculate the work of compression, Wa, according to the two step reaction for 100 ml of starting mixture and calculate

the difference in the work of compression between the reactions 1. and 2.

Assume for calculations a complete reaction at constant pressure. Temperature remains constant at 500 K, ideal gas

behaviour is assumed.

To produce hydrogen for the synthesis of ammonia, a mixture of 40.0 mol CO and 40.0 mol of hydrogen, 18.0 mol of

carbon dioxide and 2.0 mol of nitrogen are in contact with 200.0 mol of steam in a reactor where the conversion

equilibrium

CO + H O CO + H2_

2 2 is established.

c) Calculate the number of moles of each gas leaving the reactor.

IChO Paris 1990

a) Show that burning zinc sulphide in air according to

solid 2 solid 2 (gas) r (1350K)-1ZnS + 1.5O ZnO + SO H = - 448.98 kJmol

is self-sustaining at 1350 K, i.e that the heat produced is sufficient to bring the reactants from ambient temperature tothe heat temperature.

b) Starting with a stoichiometric mixture of one mole zinc blende only containing ZnS and a necessary quantity of air at

298 K, calculate the temperature to which the mixture would raise at 1350 K under standard pressure. Is the reaction

self-sustaining? Air is considered to be a mixture of oxygen and nitrogen in a volume ration equal to 1:4.

c) In fact, zinc blende is never pure and always mixed with a gangue consisting of silica SiO2. Assuming that the gangue

does not react, calculate the minimum ZnS content of the mineral so that the reaction would be self-sustaining. Give

the answer is grams of ZnS per hundred grams of zinc blende.

Data:

Standard molar heat capacities (in J K-1 mol-1):

ZnS(solid): 58.05 ZnO(solid): 51.64


20/40

SO2(gas): 51.10 O2(gas): 34.24

N2(gas): 30.65 SiO2(solid): 72.50

Molar masses (in g mol-1

): ZnS: 97.5

SiO2: 60.1

IChO Paris 1990

Nitramid NO2NH

2decomposes slowly in aqueous solution according to

2 2 2 (g) 2NO NH N O + H O

with the experimental kinetic law

a) What is the apparent order of reaction in a buffered solution ?

mechanism 1:

NO2NH2k1 N2O + H2O (rate limiting step)

mechanism 2:

NO2NH2 + H3O+ k k2 2/

/

NO2NH3+ + H2O (rapid equilibrium)

NO2NH3+ k3

N2O + H3O

+(rate limiting step)

mechanism 3:

NO2NH2 + H2O k k4 4//

NO2NH- + H3O

+(rapid equilibrium)

NO2NH- k5 N2O + OH

-(rate limiting step)

H3O+

+ OH- k6 2 H2O (very fast reaction)

b) Which of the following mechanisms is the most appropriate for the interpretation of this kinetic law. Justify your

answer.c) Show the relationship between the experimentally observed rate constant and the rate constants in the selected

mechanism.

d) Show that hydroxyl ions catalyze the decomposition of nitramid.

e) The decomposition is studied in a buffered solution of constant volume V at a constant temperature by measuring

the partial pressure of the gas N2O, insoluble in water, in a constant volume of the same value V above the solution.

The following results are obtained:

t(min) 0 5 10 15 20 25

p(Pa) 0 6800 12400 17200 20800 24000

After a sufficiently long time, the pressure stabilises at 40000 Pa. Express the pressure P as a function of time and the

constant k such that k = k / [H3O+]. Verify graphically that the kinetic law is confirmed by these experimental results.Calculate k, giving its units.


21/40

IChO Lodz 1991

The energy of stable states of the hydrogen atom is given by: En = -2.18 10-18

n-2

J-1

where n denotes the principal

quantum number.

a) Calculate the energy differences between n = 2 (first excited state) and n = 1 (ground state) and between n = 7 and n

= 1.

b) In what spectral range are the Lyman series lying ?

c) Can a single photon emitted in the first and/or sixth line of the Lyman series ionize

1) another hydrogen atom in its ground state ?

2) a copper atom in the Cu crystal ?

The electron work function of Cu is Cu = 7.44 10-19

.

d) Calculate the de Broglie wavelength of the electrons emitted from a copper crystal when irradiated by photons from

the first line and the sixth line of the Lyman series.


The paramagnetic gas NO2 can dimerize to give the diamagnetic gas N2O4: 2 NO2 (g) N2O4 (g)

a) With a diagram, show the bonding present in NO2 (g) using the concept of resonance if necessary. Use dots to

represent electrons.

b) Show, with bonding diagrams, how two molecules of NO2 (g) combine to give a molecule of N2O4 (g).

c) At 298 K, the Go

of formation for N2O4 (g) is 98.28 kJ, whereas for NO2 (g) it is 51.84 kJ. Starting with one mole of

N2O4 (g) at 1.0 atm and 298 K, calculate what fraction will be decomposed if the total pressure is kept constant at 1.0

atm and the temperature is maintained at 298 K.

d) If Ho

for the reaction N2O4 (g) 2 NO2 (g) is 53.03 kJ, at what temperature would the fraction of N2O4 decomposedbe double the one calculated in part c) ?

e) The dissociation of N2O4 (g) to give NO2 (g) is a first order process with a specific rate constant of 5.3 104

s-1

at 298

K. Starting with an initial concentration of 0.10 M, how many seconds would it take for 20 % of the original N2O4 to

decompose ?

f) The association of NO2 (g) to give N2O4 (g) is a second-order process with a specific rate constant of 9.8 106

L mol-

1s-1

at 298 K. Calculate the concentration equilibrium constant, Kc, at 298 K for the reaction 2 NO2 (g) N2O4 (g)


The concentration of carbon dioxide in the atmosphere has increased substantially during this century. The [CO2] is

expected to be about 440 ppm (440 10-6

atm) in the year 2020.

a) Calculate the concentration, in mol/l, of CO2 dissolved in distilled water equilibrated with the atmosphere in the year

2020. What is its pH-value ?

b) Calculate the enthalpy of reaction between CO2 (aq) and H2O.

c) Will the pH increase or decrease if the temperature of an equilibrated solution of CO2 in water is increased and the

concentration of dissolved CO2 maintained constant ?

Data:

Henrys Law constant for CO2 at 298 K: 0.0343 l mol-1

atm-1

Thermodynamic values, in kJ/mol at 298 K are:

Gof

Hof

CO2 (aq)-386.2 -412.9

H2O (l) -237.2 -285.8

HCO3- (aq) -587.1 -691.2H

+(aq) 0.00 0.00


22/40

IChO Peruggia 93

Poisoning by carbon monoxide produced by ill-adjusted heating systems is a serious problem. In a methane burner the

following reactions occur:

CH4 O2 Co2 CO H2O

Hof(kJmol-1) -74.9 0 -393.5 -110.5 -241.8

So (JK-1mol-1) 186.2 205.0 213.6 197.6 188.7

a) Calculate the equilibrium constants of both reactions at T = 1500 K, assuming that the values ofHo an d So

are independant of temperature.

b) Find the relationship between the mole numbers of oxygen and carbon monoxide at equilibrium (T = 1500 K, P =

1 atm) when air is admitted into the burner in such amount that the mole ration CH4/O2 is 2:1. Assume the following

volume composition for air: 80% N2 and 20% O2. Make the approximation that nCH4 = 0 at equilibrium and justify

it on the basis of the answers to question a).

c) Calculate the equilibrium mole fraction (x) of CO in the conditions given in b). Assume that the mole fraction of

CO is very small in comparison with that of CO2 and consequently the total number of molesdoes not appreciably vary

during the combustion. Justify this assumption.

d) Repeat the calculations of items b) and c) assuming that twice the amount of air (compared to the

conditions of questions b) is admitted into the burner (4 mol O2 / 1 mol CH4).e) Calculate the concentrations of CO (expressed as ppm by volume) in the fumes after condensation of w a t e r

vapour under the conditions of questions b) and d), assuming that the composition does not change during the

cooling from 1500 K to room temperature and neglecting the contributions of water vapour in the composition of the

gas phase.

IChO Oslo 1994

Platinum(IV) oxide is not found in the nature, but can be prepared in a laboratory. Solid platinum(IV) oxide is in equilibrium

with platinum metal and oxygen gas at 1 atm (101.3 kPa) at 650 C.

a) This suggests that the conditions on the Earth, when platinum minerals were formed, were:

1. pO2= 1 atm, t = 650 C;

2. pO2< 1 atm, t < 650 C;

3. pO2> 1 atm, t < 650 C;

4. pO2< 1 atm, t > 650 C;

5. pO2> 1 atm, t > 650 C

Mark the most probable altemative [ 1 ] - [ 5 ] on the answer sheet.

b) What are G and Kpfor the formation of platinum(IV) oxide at 1 atm oxygen pressure and 650 .C?

The preparation of platinum(IV) oxide involves boiling a solution which contains hexachloroplatinate(IV) ions with

sodium carbonate. In this process PtO2.nH2O is formed and is subsequently converted by filtration and heat treatmentto platinum(IV) oxide. In the following we assume n=4. PtO

2.4H

2O (or Pt(OH)

4.2H

2O) can be dissolved in acids and

strong bases.

c) Write the balanced equations for the preparation of platinum(IV) oxide according to the procedure given above.

d) Write the balanced equations for the dissolution of PtO2.4H

2O in both hydrochloric acid and sodium hydroxide.

Platinum is mainly found in nature as the metal (in a mixture or alloyed with other precious metals). Hexachloroplatinate(IV)

ions are formed by dissolving platinum in aqua regia. Aqua regia is a mixture of concentrated hydrochloric and nitric

acids in proportion 3:1. Upon mixing, this forms nitrosyl chloride (NOCl) and atomic chlorine. The latter is believed to be

the active dissolving component.

The hexachloroplatinate(IV) ions can be precipitated as diammonium hexachloroplatinate(IV). By thermal decomposition

of this compound, finely powdered platinum and gaseous products are formed.

e) Write the balanced equations for the formation of aqua regia and its reaction with platinum.

f) Write the balanced equation for the thermal decomposition of diammonium hexachloroplatinate(IV) at elevated


23/40

temperature.

From diammonium hexachloroplatinate(IV) we can prepare Pt(NH3)

2Cl

2which occurs in the cis (H

fo= -467.4 kJ/mol,

Gfo

=-228.7 kJ/mol) and trans Hfo = -480.3 kJ/mol, Gfo

= -222.8 kJ/mol) forms.

g) The occurrence of the isomers shows that the geometry of Pt(NH3)

2Cl

2is

1. linear ?2. planar ?

3. tetrahedral ?

4. octahedral ?

h) Indicate whether the cis or trans isomer is thermodynamically more stable. If you choose cis, mark [ 1 ] on the answer

sheet. If you choose trans, mark [ 2 ].

Platinum is used as a catalyst in modern automobiles. In the presence of the catalyst, carbon monoxide Hfo

= -110.5 kJ/

mol, Gfo

= -137.3 kJ/mol) reacts with oxygen to form carbon dioxide Hfo

= -393.5 kJ/mol, Gfo

= -394.4 kJ/mol).

I) Is the reaction spontaneous at 25 C? [ 1 ] yes or [ 2 ] no. Is the reaction endothermic or exothermic? If you choose

endothermic, mark [ 1 ] on the answer sheet. If you choose exothermic, mark [ 2 ]. Calculate So for the reaction.Determine whether the entropy of the reaction system [ 1 ] increases or [ 2 ] decreases.

j) Derive an expression for the temperature dependence of the equilibrium constant in this case.

The overall catalytic reaction is simple, whereas the reaction mechanism in the gas phase is very complicated, with a

large number of reaction steps. With platinum as the catalyst the significant reaction steps are:

1. (i) adsorption of CO and adsorption/dissociation of O2([H = -259 kJ per mol CO + O),

2. (ii) their activation (105 kJ per mol CO + O) and

3. (iii) the reaction to and the desorption of CO2(H = 21 kJ per mol CO

2).

A one-dimensional energy-diagram for the platinum catalyzed oxidation of carbon monoxide to dioxide can be represented

as:

[Image]

k) Mark the correct alternative of [ 1 ] - [ 4 ] on the answer sheet.

IChO Oslo 1994

a) When an ideal, monatomic gas expands reversibly from volume V1to volume V

2, work

w PdVV

V

= 1

2

is performed on the system by the surroundings. In this equation, w is the work and P is the pressure of the gas.

Determine the work performed when one mole ideal gas expands isothermally from V1 = 1.00 dm3 to V2= 20.0 dm3 at

temperature T = 300.0 K. Given: The gas constant R = 8.314 J K-1 mol-1

b) Determine how much heat must be added to the gas during the process given under a).

c) The gas will perform less work in an adiabatic expansion than in an isothermal expansion. Is this because the

adiabatic expansion is characterized by (check the square you think is most important).

1. The volume of the gas is constant

2. The expansion is always irreversible

3. No heat is supplied to the gas

d) The cyclic process shown schematically in Figure 1 shows the four steps in a refrigeration system with an ideal gasas working medium. Identify the isothermal and adiabatic steps in the process. Here, T

Hand T

Crepresent high and

low temperature, respectively. Specify for each step whether it is adiabatic or isothermal.


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4. Technical chemistry

IChO Leiden 1986

The equilibrium constant of the reaction A(g) + 2 B(g) 2 C(g) is Kp = 10.0 MPa-1. The starting materials are suppliedat 25 C and heated to 100 C where complete equilibration takes place. Below 100 C the reaction rate is negligibly small.The whole process is executed continuously in a stationary state. The boiling points at 0.1 MPa of A, B and C are 40 C,80 C and 60 C, respectively. The three compounds have the same heat of evaporation: q J/mol. The heat capacities ofA,B and C may be neglected. A schematic diagram of a distillation is shown below (Fig. 4).

bottom product, liquid

top product, liquid

Fig. 4 Schematic diagram of a distillation

The total heat used at each of the two distillations is 3q J/mol (of the top product). Apart from distillation columns (each

with its own evaporator and condenser) the pieces of apparatus shown of Fig. 5 are available.

R reactor

heater/evaporato

cooler/condenser


25/40

Fig. 5 Pieces of apparatus available

a) Draw a flow diagram of the process in which all flows are given (flow sheet) and in which the starting materials are

used as efficiently as possible using as few pieces of apparatus as possible.

b) Express the equilibrium constant in terms of the degree of conversion and of total pressure for the case that the feed

to the reactor is stoichiometric. What is the value of the degree of conversion when total pressure is 0.100 MPa?

c) Number all flows. Calculate the composition of each flow in mol/s for a rate of production of 1 mole of C per secondunder the conditions of part b.

d) In what respect can the reaction conditions influence the composition of the mixture that leaves the reactor? (Refer

to question b)

e) The process requires energy. For the conditions of b explain where energy should be supplied in the flow diagram

drawn in part a. Derive an expression for the total energy needed.


Treating waste water in a sewage plant, 45 % of its carbohydrate (CH 2O)n is completely oxidized, 10 % undergoes

anaerobic decomposition by fermentation (two components) and the rest remains in the sludge. The total gas formation

is 16 m3

per day (25 C, 100 kPa).

a) What is the amount of carbohydrate remaining in the sludge measured in kg per day ?

b) Given the heat of combustion of methane (- 882 kJ / mol), calculate the amount of energy that can be produced by

combustion of the methane formed.

c) Knowing the concentration of the carbohydrate in the waste water to be 250 mg/ l, calculate the daily amount of waste

water processed in the plant in m3

of water per day.

IChO Halle 89

Sulphur dioxide is removed from waste gases of coal power stations by washing with aqueous suspensions of calcium

carbonate or calcium hydroxide. The residue formed is recovered.

a) Write all reactions as balanced equations.

b) How many kilograms of calcium carbonate are daily consumed to remove 95% of the sulphur dioxide if 10000 m3/h of

waste gas (corrected to O C and standard pressure) containing 0.15% sulphur dioxide by volume are processed?How many kilograms of gypsum are recovered thereby ?

c) Assuming that the sulphur dioxide is not being removed and equally spread in an atmospheric liquid water pool of

5000 m3

and fully returned on earth as rain, what is the expected pH of the condensed water ?

d) If a sodium sulphite solution is used for absorption, sulphur dioxide and the sulphite solution can be recovered.

Write down the balanced equations and point out possible pathways to increase the recovery of sulphur dioxide

from the liquid phase.

Information

Protolysis of sulphur dioxide in aqueous solution is described by:

2 2 3+

3-SO + 2 H O H O + HSO

The acidity constant Ka is equal to Ka = 10-2.25

mol/l. Assume ideal gases and a constant temperature of 0 C at standardpressure. M[CaCO3] = 100 g/mol; M[CaSO4] = 172 g/mol

IChO Paris 1990

Hydrogenation of benzene to cyclohexane is performed in continuous reactors. The reaction is as follows:

6 6 2 6 12 0C H + 3H C H H = - 214kJ / mol at 200 C


26/40

Two different types of reactors are examined: continuous plug flow reactor and continuous stirred reactor (Fig. 6): In the

continuous plug flow reactor, reactants are introduced and mixed in the inlet of the reactant (F1, F2); each slice of

reaction medium (grey shaded zone in the diagram) moves along the inside without mixing with the others and exits from

the reactor (F3). When the flow has stabilized, concentrations and conditions are time-independent but dependent on

the location in the reactor. In the continuous stirred reactor, stirring is perfect and instantaneous. When the flow is

stabilized, concentration and conditions are time-independent and identical at every point of the reactor.

Fig. 6 Continuous plug flow and continuous stirred reactor

The differential yield of hydrogenation (Y) is given by

Y = - d([C6H12])/d([C6H6]).

The proportion of benzene already hydrogenated is given by p = [C6H12]/([C6H12]+[C6H6]). The relationship between

p and Y is shown in Fig. 7.

The aim is to hydrogenate almost all the benzene, i.e. 0.9 p 1 with the best mean yieldY = - ([C6H12])/([C6H6]).

a) What is the value of p in a continuous stirred reactor leading to the maximal mean yield of hydrogenation? What is

the maximal value of Y?b) For a continuous plug flow reactor, give a rough value of Y for 0.9 p 1:Y = 0.97, 0.98, 0.99 or 1.00 ?

c) Represent by grey shading the quantities of benzene lost in installation P (plug flow reactor) and installation MP

(stirred reactor + plug flow reactor).

d) Give the amount of benzene annually lost in installation P and MP assuming an annual consumption of 100000 tons

of benzene.


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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.96

0.97

0.98

0.99

1.00

y

[C6H12]/([C6H12]+[C6H6])

Fig. 7 relationship between p and Y

IChO Lodz 1991

Sulphuric acid is produced by catalytic oxidation of SO2 to SO3, absorption of SO3 in concentrated sulphuric acid

forming oleum (containing 20% SO3 by mass) and appropriate dilution hereafter. The gas leaving the catalyst chamber

contains nitrogen, oxygen, a trace of SO2 and 10 % (by volume) of SO3.

SO3 is converted into sulphuric acid (98 % by mass) and/or oleum.

a) Assuming that oleum is the only product formed, calculate the mass of water which is required for 1000 m3

of gas

leaving the catalyst chamber (273 K, 101.3 kPa) and determine the mass of oleum produced hereby.b) Assuming that only 98 % sulphuric acid is produced, calculate the necessary mass of water and the mass of product

produced thereby.

c) In the industry both 98 % sulphuric acid and oleum are produced in a mass ratio of x = m1/m2, where m1 denotes the

mass of oleum, m2 the mass of 98 % sulphuric acid. Find an expression y = f(x) to describe the relation between the

mass of water consumed for 1000 m3

gas. Show that the results of a) and b) are in good agreement with your

mathematical expression.


28/40

5. Organic chemistry

IChO Leiden 1986

The high efficiency of catalysis by enzymes is mainly due to an enzyme-reactant complex in which the reacting group is

placed in a favourable position for the reaction with respect to the catalyzing groups of the enzyme. Studies are carried

out with model compounds in which a catalyzing group has been introduced in the proximity of the reaction centre of the

molecule. As an example we consider the hydrolysis of the amide bond in compound A. This reaction proceeds in waterat 39 C and pH = 2 more than a million times faster than the hydrolysis of compound B.

OH

O

O

NH

O

NH

O

HO O

O

N

A B C

0 1 2 3 4 5 6 7 p

-3

-2

log k

hyd

Fig. 8 Relation between the rate constant khyd and pH for the hydrolysis of A

The relation between the rate constant khad and pH for the hydrolysis of A at 39 C is shown in figure 8.


29/40

Further observations

Addition of water to the iso-imide C gives a rapid reaction, which initially yields A. Subsequently, hydrolysis of A

occurs.

The amid carbonyl group in A is labelled with13

C and the hydrolysis is allowed to take place in H218

O at pH = 2 and 39

C. The diacid formed upon hydrolysis is isolated, converted into the di-silver salt and completely decarboxylated withbromine in a anhydrous reaction medium. The formed carbon dioxide is a mixture of particles of masses 44, 45, 46 and 47

which are formed in equal amounts.

a) Why is the hydrolysis of A so much faster then that of B ?

b) Explain why the rate of hydrolysis of A is independent of pH on the range between pH = 0 to pH = 2.

c) Why does khyd decrease so rapidly at pH-values higher than 3.

d) Give a detailed reaction mechanism for the hydrolysis of A. Indicate which step in the reaction is rate determining.

e) Show that the observations further made are consistent with the reaction mechanism given under d.

IChO Leiden 1986

Bacterial conversion of saccharose leads to (S) - (+) - 2 - hydroxypropanoic acid (L - (+) - lactic acid), which forms a cyclicester out of two molecules. This dilactide can be polymerized to a polylactide, which is used in surgery.

a) Give the spatial structures and Fischer projection of L - (+) - lactic acid and its dilactide

b) Sketch the structure of the polylactide discussed above (at least three units). What is its tacticity (iso-, syndio- or

atactic) ?

c) Draw the isomeric dilactides formed out of racemic lactic acid. Show the configuration of the chiral centres.

L - (+) - lactic acid is used for the preparation of the herbicide Barnon that is used against wild oats. In this case (+) - lactic

acid is esterified with 2 - propanol and then the hydroxyl group is treated with methanesulfonyl chloride. The product is

then submitted to a SN2 - reaction with 3 - fluoro - 4 - chloro - phenylamine, where the methanesulfonate group leaves as

CH3SO3-. Finally a benzoyl group is introduced with the help of benzoyl chloride.

d) Draw the Fischer projection of the various consecutive reaction products.


Write the structural formulae of the compounds A to I in the following reaction sequence.

A B C D EMg/abs. ether 1.

O

2. H2O

PBr3 NaCN

E F G HH2O/H2SO4

heat

SOCl2 Friedel-Crafts

AlCl3I

H2

cat.

I

conc. H2SO4

heatindene


30/40

IChOVeszprem - Budapest 1987

a) What ratio of primary / secondary / tertiary products can statistically be expected in the high temperature chlorina-

tion of methylbutane? Assume that the reaction rate of the substitution is equal for all C-H bonds.

b) Which of the following alcohols: pentan-1-ol, pentan-2-ol and 2-methyl-butan-2-ol react with the following reagents

listed below ?1) cold, concentrated sulphuric acid; 2) CrO3/H2SO4; 3) ZnCl2/HCl; 4) I2/NaOH

d) Which of the following aldohexose structures (Fig. 9) are 1) enantiomer pairs, 2) diastereomer pairs?

e) Two hydrocarbon compounds of the same molecular formula C4H8 easily react with hydrogen in presence of a

platinum catalyst. During ozonolysis only ethanal (acetaldehyde) is formed in both cases.

The1H-NMR spectra of the two compounds show two types of protons in a ratio of 3 to 1. Draw the structures of the

described compounds.

f) Select the components of the group listed below that can be obtained during complete hydrolysis of lecithin (a

phospholipid): serine, phosphoric acid, sphingosine, choline, glycerol, myo-inositol,

phosphatidic acid and fatty acids).Which of the following carboxylic acid can be formed in the tricarboxylic acid

cycle (Krebs citric acid cycle): maleic acid (cis-butenedioic acid), mandelic acid (a-hydroxi-phenylethanoic acid),

malic acid (2-hydroxy-butanedioic acid), tricarb-allylic acid (propane-1,2,3-tricarboxylic acid), oxalacetic acid

(2-oxo-butanedioic acid), keto-glutaric acid (2-oxo-pentanedioic acid), fumaric acid (trans-butenedioic acid) and

acetoacetic acid (3-oxo-butanoic acid).

g) To which structures is the nicotinamide moiety (part) of NAD+

converted during the uptake of hydrogen to form

NADH (A, B or C) ?

CH2OH

CHO

OH

OH

HO

OH

CH2OH

CHO

OH

OH

HO

CH2OH

CHO

CH2OH

CHO

OHHO HO

OH

HO

HOHO

HO

OH

A B C D

Fig. 9 Aldohexose structures

NH+ N N

CONH2CONH2CONH2

+ H+

+ H+

A B C

IChO Helsinki 1988

A common compound A is prepared from phenol and oxidized to compound B. Dehydration of A with H2SO4 leads to


31/40

compound C and treatment of A with PBr3 gives D. In the mass spectrum of D there is a very strong peak at m/e = 83 (base

peak) and two molecular ion peaks at m/e 162 and 164. The ratio of intensities of the peaks 162 and 164 is 1.02. Compound

D can be converted to an organomagnesium compound E which reacts with a carbonyl compound F in dry ether to give

G after hydrolysis. G is a secundary alcohol with the molecular formula C8H16O.

a) Outline all steps in the synthesis of G and draw the structural formulae of the compounds A- G.

b) Which of the products A - G consist of configurational stereoisomeric pairs ?

c) Identify the three ions in the mass spectrum considering isotopic abundances given in the text.

IChO Helsinki 1988

Upon analyzing sea mussels a new bioaccumlated pollutant X was found as determined by mass spectroscopy coupled

to a gas chromatograph. The mass spectrum is illustrated in Fig 10. Determine the structural formula of X assuming that

it is produced out of synthetic rubber used as insulation in electrolysis cells that are used for the production of chlorine.

Give the name of the compound X. The isotopic abundances of the pertinent elements are shown in Fig. 11. Intensities

of the ions m/e = 196, 233, 268 and 270 are very low and thus omitted. Peaks of the13

C containing ions are omitted for

simplicity

0

50

100

180 190 200 210 220 230 240 250 260

M+

Element Mass Abundance Mass Abundance Mass Abundance

H 1 100.0 2 0.015

C 12 100.0 13 1.1

N 14 100.0 15 0.37

O 16 100.0 17 0.04

P 31 100.0

S 32 100.0 33 0.80 34 4.4

Cl 35 100.0 37 32.5

Br 79 100.0 81 98.0

Fig. 10 Mass spectrum and isotopic abundances

IChO Halle 1989

A chemically and biologically important class of organic compounds are carboxylic acids.

a) Draw the structural formulae of all isomeric cyclobutanedicarboxylic acids and give the corresponding systematic

names.

b) There are three stereoisomers, I,II and III. Draw perspective stereoformulas of I, II and III indicating the relative

configuration of each molecule.

c) Which pairs of stereoisomers I,II and III are diastereoisomers and which are enantiomers of each other ?

d) Which reaction can be used to determine the relative configuration of diastereoisomers?


32/40

e) How may the enantiomers of cyclobutane-1,2-dicarboxylic acid be separated ?

f) Indicate the absolute configuration of each asymmetric centre on the structures of the stereoisomers I, II and III

using the Cahn-Ingold-Prelog rules (R,S system).

Fats (lipids) contain a non-polar (hydrophobic) and a polar (hydrophilic) group.

g) Draw the structures of Z-octadec-9-enoic acid (oleic acid), octadecanoic acid (stearic acid) and hexadecanoic acid

(palmitic acid). Using the these three fatty acids draw a possible structure of a triacyl glyceride.h) Write the equations for the hydrolysis reactions of your triacyl glyceride in NaOH(aq). Give the corresponding

mechanism.

i) Which of the following fatty acids, C21H43COOH, C17H35COOH or C5H11COOH is the least soluble in water.

k) Phospholipids are an important class of bio-organic compounds. Draw the structure of the phophatic acid derived

from your triacyl glyceride and mark the hydrophilic and hydrophobic groups.

l) Draw two possibilities for the association of six identical molecules of phopholipids in water.

m) Biomembranes consist of a phospholipid bi-layer. Draw such a structure. What other biomacromolecules are contained

in such biomembranes?

IChO Paris 1990

Synthesis of Haloperidol, a powerful neuroleptic

a) Give a simplified equation for the preparation of methyl 4 - chlorobenzoate starting from benzene and all necessary

inorganic substances. Diazomethane must be used in your synthesis.

b) How can -butyrolactone be converted into 4-hydroxybutanoic acid (K)? Convert K into 4-chlorobutanoyl chlorid(L)!

Methyl 4 - chlorobenzoate is treated with an excess of vinylmagnesiumbromide in anhydrous ether. M is obtained after

hydrolysis. Treating M with an excess of HBr in anhydrous conditions in the presence of benzoyl peroxide, N is

obtained. N reacts with ammonia to form 4-(4-chlorophenyl)-4-hydroxypiperidine (O).

c) Write down the structure of M, N and O and indicate the mechanism of the reaction leading to M.

d) In the presence of moderate amounts of aluminium chloride, L reacts with fluorobenzene to yield mainly a ketone P(C10H10OFCl). Sketch the structure of P and indicate the mechanism.

e) Give a chemical and physical test method for the determination of the carbonyl group. How can you make sure that

the carbonyl group does not belong to an aldehyde group?

f) P reacts with O on a 1 to 1 mole basis, in basic media, to give H that contains only one chlorine atom on the aromatic

ring. Give the structure of H which is haloperidol.

g) State the multiplicity of each resonance in the1H NMR spectrum of K. Assume that all coupling constants between

protons and adjacent carbons are identical.

IChO Lodz 1991

A polymer X contains 88.25% C and 11.75% H. In dilute solutions it reacts with bromine and ozone. The thermal

degradation produces a volatile liquid Y in 58% yield. Y starts boiling at 34 C and contains 88.24% C and 11.75% H.Some higher boiling products are formed both from the cracking of X and from Diels-Alder-type cyclization of Y. The

vapour of Y was 34 times as dense as hydrogen. The product of bromination yields Y containing 82.5% bromine by mass.

Ozonolysis of Y followed by mild reduction gave A and B in a molar ration A:B = 2:1. Only compound B gave a positive

result for the iodoform reaction.

a) Determine the molecular formula and the molar mass of A, B and Y and show its structure.

b) Give an equation for the reaction of Y with bromine.

In the catalytic hydrogenation of 13.6 g of X, 0.2 mole of hydrogen was absorbed. Ozonolysis of X followed by mild

reduction yielded compound Z (80.0% C, 8.0% H).

c) Give the molecular formula of Z and the degree of unsaturation of X.

Compound Z gave a positive reaction with Fehlings solution. Mild oxidation of Z gave an acid C of which a solution was


33/40

titrated with aqueous KOH. 0.116 g of C neutralized 0.001 mol KOH.

In the iodoform probe 2.90 g of C yielded 9.85 g of iodoform. In addition the alkaline filtrate yielded compound E upon

acidification.

d) What is the molar mass of C and what functional groups are present in Z?

When heated, E loses water to form F. Both E and F react with an excess of acidified ethanol to G (C8H14O4).

e) Sketch the structures of C, E, F, G and Z and give an unbalanced reaction scheme for EFG.

X exists in isomeric forms which are stereoregular. Show the structure (containing at least 3 monomer units) for two

possible stereoisomers of X.

IChO Lodz 1991

After passing 0.25 mol of hydrocarbon A over heated pumice (950 K) in an iron pipe, compound B in yield of 80 % (i.e.

15.4 g) and hydrogen, 2.4 l (295 K, 102 kPa) were obtained; B and hydrogen are the only products. A mixture of halogen

derivatives C, D, E, F and G is produced from B by reaction with a halogen in precence of a Lewis acid. C to G contain each

one halogen atom more then the preceeding compound. For the compounds C to F only one of the possible isomers is

formed. In compound G there is no such preference and its three isomers G1, G2 and G3 are found in the mixture.Compounds C to F easily racemize, so that no optical isomerism occurrs. However racemization is difficult for G1, G2 and

especially for G3. In the mass spectrum of E only three isotopic peaks were observed. Their relative intensity is 1: 1: 0.3.

Information:

- kortho > kpara in compound B

- the effect of the first halogen in the ring: kpara > kortho- compounds D and F have a center of symmetry

- the contribution of carbon and hydrogen isotopes in the mass spectrum of E are negligible

- natural abundance of halogen isotopes:19

F = 100 %35

Cl = 75.53 %

37

Cl = 24.47 %79Br = 50.54 %

81Br = 49.46 %

127I = 100 %

a) Give the structural formulas of A, B, C, D, E, F, G1, G2 and G3. Explain your choice for the halogen.

b) Draw and label stereo-formulas of the rotational isomers of D for = 0, /2, 3/2 where denotes the dihedral ortorsional angle in radians and = 0 describes the configuration with maximal energy.

c) Draw profiles of the energy changes as a function of the angle of rotation around the C-C bond for compounds C and

D.

d) Give G1, G2, G3 in their increasing difficulty of racemization. Explain your answer.

e) Draw stereo-formulas of the enantiomers of G3.

f) Suggest a chemical reaction or a biological method by which compounds like these can be destroyed.


Coniferyl alcohol which is not soluble is water or aqueous NaHCO3 has the molecular formula C10H12O3. It is isolated

from pine trees. A solution of Br2 in CCl4 is decolorized when added to coniferyl alcohol forming A (C10H12O3Br2). Upon

reductive ozonolysis vanillin (4-hydroxy-3-methoxybenzaldehyd) and B (C2H4O2) are produced. Coniferyl alcohol

reacts with benzoyl chloride (C6H5COCl) in the presence of a base to form C (C24H20O5). This product rapidly decolorized

KMnO4 (aq) and is insoluble in dilute NaOH.

Coniferyl alcohol reacts with cold HBr to form D (C10H11O2Br). Hot HI converting ArOR to ArOH and RI converts

coniferyl alcohol to E (C9H9O2I) and CH3I. In an aqueous base CH3I and coniferyl alcohol form F (C11H14O3), which is

not soluble in a strong base, but decolorizes Br2/CCl4-solution.

a) Draw the structures of compounds A-F including coniferyl alcohol.

b) There are a number of stereoisomers of A. Draw Fischer projections of all stereoisomers labelling each chiral centre

with the proper R or S designation giving the absolute configuration about the chiral centre.


34/40


Rose oil is obtained from steam distillation of roses. It contains a number of terpenes, one of which is geraniol, C10H18O

(A). Upon oxidation, geraniol can either give a ten-carbon aldehyde or a ten-carbon carboxylic acid. Reaction with two

moles of bromine gives a tetrabromide (C10H18OBr4) (B). Geraniol reacts with HBr to give two bromides of formula

C10H17Br.

When geraniol is vigorously oxidized, three products are obtained:

O

HO

O

O

OH

OH

O

O

a) Give the structure of geraniol (A), B and the two bromides of formula C10H17Br

b) Indicate which of the two bromides is formed in greater proportions.


Complete the following reactions and indicate their stereoselctivity (cis/trans) where appropriate.


35/40

O

O

O

O

O

OH

isoamyl acetate, bee alarm pheromone

+ NaOH

I2 / NaOH

NaBH4

CH3CH2OH

CH3CH2MgBr H3O+

2-heptanone, ant alarm pheromone

a)

b)

O3

Zn, H3O+

green peach aphid

pheromone

O3

H2O2, H3O+

c)

H2O / H2SO4

d)

Na2Cr2O7, H2SO4

CH3COOH


36/40

Br2/CCl4

KMnO4-, OH-

cold

H3O+

HBr

peroxide

BH3. ether

H2O2, OH-

H2, Pt

e)

(i)

(ii)

(iii)

(iv)

(v)

(vi)

IChO Peruggia 93

Frontalin is a pheromone of the western pine beetle with a composition C 67.58%, H 9.92%, O 22.50%. It is an acetal that

can be prepared starting from sodium diethylmalonate and 3-chloro-2-methylpropene. The resulting product A is

hydrolized by conc. potassium hydroxide and then decarboxylated by treatment with hot acetic acid to give a compound

B. The latter reacts with aq. NaHCO3 (gas evolution is observed) as well as with cold aq. KMnO4 which turns to a brown

color. The compound B is then converted by LiAlH4 into C (C6H12O).

Treatment ofC with p-toluenesulfonyl chloride in pyridine and then sodium cyanid in dimethylsulfoxide affords D

(C7H11N). The subsequent reaction of D with methylmagnesium iodide, followed by hydrolisis, yields E (C8H14O). E

shows IR absorption at ca. 1700 cm-1. The epoxidation of E with metachloroperbenzoic acid then affordsF (C8H14O2)

which, by treatment with diluted acids is converted to frontalin acetal G.

Question: Draw the structure of compounds A-G.

IChO Peruggia 93

By treatment with a optically active hydroborating agent (R2*BH) and subsequent oxidative work-up, 1-

methylcyclopentene affords a mixture of aclohols that rotates the plane of polarized light. [H.C.Brown,J.Org.Chem., 47,

5074, 1982]

a) Write the structures of the alcohols.

b) Assign the configurations (R,S) to the chiral centers.

c) Explain in a sentence why the obtained mixture of alcohols exhibits optical activity.

IChO Oslo 1994

a) What is the correct systematic name (IUPAC name) for the compound below?

(CH3)

2CHCH(CH

2CH

3)(CH

2CH

2CH

3)

1. 3-Isopropylhexane

2. 2-Methyl-3-propylpentane3. Ethyl isopropyl prop

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